In general, a speed ratio, a torque capacity and so on of a vehicular transmission is changed hydraulically. For example, a control unit for a belt-driven continuously variable transmission is described in Japanese Patent Laid-Open No. 2011-163393. The control unit is comprised of a pressure delivery valve for delivering an oil pressure to an oil chamber of a drive pulley, a pressure drain valve for draining an oil pressure from the oil chamber, a pressure delivery valve for delivering an oil pressure to an oil chamber of a driven pulley, and a pressure drain valve for draining an oil pressure from the oil chamber. Specifically, a poppet valve actuated by a magnetic solenoid is used as those valves. Each valve is individually comprised of a valve element having a tapered or semispherical nose portion, a valve seat to which the valve element is contacted, a plunger attached to the valve element, a spring for pushing the valve element onto the valve seat, and an electromagnetic coil for pulling the plunger from the valve seat against an elastic force of the spring. An input port is formed in the valve seat, and an output port is formed at a site where the valve element is situated. The input port of each valve is closed by pushing the valve element onto the valve seat by the elastic force of the spring. By contrast, the input port of each valve is opened by isolating the valve element away from the valve seat against the elastic force of the spring by the electromagnetic force of the electromagnetic coil, thereby providing a communication between the input port and the output port.
Japanese Patent Laid-Open No. 2010-266034 describes a controller for continuously variable transmission comprised of a supply valve for supplying a hydraulic fluid to an oil chamber of a primary pulley, and a supply solenoid for applying a pilot pressure to the supply valve, a discharge valve for draining the fluid from the oil chamber, and a discharge solenoid for applying a pilot pressure to the discharge valve. According to the teachings of Japanese Patent Laid-Open No. 2010-266034, the supply valve is comprised of a casing, a spool having an after mentioned valve portion held in the casing, and a spring elastically moving the spool. On the casing, there are formed an inlet port to a line pressure is applied, pilot ports to which the pilot pressure is applied, and an outlet port communicated to an oil chamber of a secondary pulley. Accordingly, the pilot pressure is applied to the spool against the elastic force of the spring. The supply valve taught by Japanese Patent Laid-Open No. 2010-266034 is adapted to provide a communication between the inlet port and the outlet port by pushing the valve portion while compressing the spring by the pilot pressure delivered from the supply valve. Given that the communication between the inlet port and the outlet port is provided, the hydraulic fluid is allowed to flow into to the hydraulic chamber of the primary pulley. By contrast, given that the pilot pressure is not delivered from the supply valve, the valve portion is moved by the elastic force of the spring to cut off the communication between the inlet port and the outlet port. In this situation, delivery of the fluid to the hydraulic chamber of the primary pulley is cutoff. Thus, the supply valve taught by Japanese Patent Laid-Open No. 2010-266034 is adapted to selectively deliver the fluid to the oil chamber of the primary pulley by altering a position of the valve portion by controlling the deliver valve.
As described, the control unit taught by Japanese Patent Laid-Open No. 2011-163393 is configured to directly open and close the port communicated with the oil chamber by the valve element actuated by the electromagnetic force and the elastic force of the spring. To this end, a high pressure in the oil chamber is directly applied to the valve element. Therefore, a large elastic force of the spring against the high oil pressure is required to keep the valve closing, and the solenoid has to generate a sufficient electromagnetic force to move the valve element in an opening direction against the elastic force of the spring. For this reason, a larger spring and a larger solenoid are required to counteract the pressure in the oil chamber thereby enlarging a size of the control system. In addition, a larger electricity is required to open the valve.
As also described, the controller taught by Japanese Patent Laid-Open No. 2010-266034 is configured to control the delivery and drainage of the oil pressure to/from the oil chamber by the spool valve. That is, oil leakage from the spool valve is worsened with a rise in the oil pressure in the hydraulic chamber. For this reason, an energy loss may be increased.
In order to deal with the disadvantages contained in the teachings of the above explained prior art documents, adaption of a balance piston valve may be effective. In the balance piston valve, a valve element integrated with a piston is actuated by a pressure deference between the positive hydraulic chamber and the back pressure chamber. However, a differential pressure may be decreased depending on an expansion and a contraction of air intruding into the oil in the back pressure chamber. Consequently, an opening response of the valve element may be worsened, that is, a control response may be worsened.